This invention concerns a polyorganosiloxane composition which is suitable for use in the manufacture of improved mica products and improved silicone mica products using the polyorganosiloxane composition. More specifically, the invention concerns mica products with superior mechanical strength, electrical characteristics, water resistance, and heat resistance, and also concerns a polyorganosiloxane composition used in the manufacture of mica products.
Mica, owing to its superior electrical characteristics and heat resistance, is widely used. Its mechanical strength, however, is poor and must be strengthened by molding or laminating with a binder for manufacturing mica products. The conventional binder is an inorganic polymer of a primary phosphate group or organic resin. One of the drawbacks of binders using inorganic polymer derived from condensed primary phosphate metal salt is a high manufacturing cost due to special facilities required for producing high temperatures of over 400.degree. C. during the laminating process. Mica products thus obtained have superior heat resistance, but another drawback is inferior water resistance, which negatively affects mechanical strength and electrical characteristics in the presence of moisture. On the other hand, the advantages of using organic resins such as alkyd resin, polyester resin, epoxy resin or acrylic resin as binders are that lamination and molding can take place at a relatively low temperature of 100.degree.-200.degree. C. and the products have good mechanical characteristics. There are disadvantages, however, such as inferior water resistance compared to those using an inorganic polymer, as well as inferior heat resistance.
Conventionally, polyorganosiloxane resins with a heat resistance superior to organic resins are used as binders for mica products. Mica products using polyorganosiloxane resins exhibit superior heat resistance compared to those using organic resins, but are inferior in mechanical characteristics and their water resistance is not always satisfactory. To improve water resistance, the concurrent use of a polyorganosiloxane resin and a carbon functional silane is recommended. For example, the method of adding a silane containing an amino group, a silane containing a ketoxime group, a silane containing an acetoxy group, or silazane compounds to polyorganosiloxane resins containing a vinyl group, and using peroxides for curing is already known (Japanese Patent Kokai 1973 No. 24295). With this method, molding and laminating processes are easy, and products molded by this method have an extremely good water resistance. However, a high consumption of an expensive silicon-bonded vinyl group resin, generally of 10-20 mole percent vinyl, renders this method commercially disadvantageous. Further, such products are inferior in heat resistance and a higher heat exposure of such products, compared with mica products using an ordinary polyorganosiloxane resin, causes extensive smoking. Another method is adding a mixture of phosphoric acid and organic silicon compounds containing an epoxy group to polyorganosiloxane resin and curing it. (Japanese Patent Kokai 1975 No. 98550). The mixture, however, is not compatible with polyorganosiloxane resin. Also, in many cases the method requires dispersion by adding a large volume of alcohol due to the water content. This may result in mica paper being damaged by water or alcohol during the processing of this mixture into laminated mica paper. Another defect is poor stability during storage due to the increased condensation of polyorganosiloxane resin by the presence of phosphoric acid in this mixture.
The method using alkyl silicate as a binder and an organic acid or inorganic acid to cure is also known (Japanese Patent Kokai 1976-Gazette No. 127106). The mica products obtained with this method are superior in heat resistance, but extremely inferior in their mechanical characteristics.
This invention eliminates these defects in the conventional methods and provides silicone mica products with superior mechanical strength, electrical characteristics, water resistance, and heat resistance. It further provides polyorganosiloxane compositions suitable as mica binder compositions.